The present invention relates generally to powertrain diagnostics conducted during skip fire control of an internal combustion engine.
Skip fire engine control is understood to offer a number of benefits including the potential of increased fuel efficiency. In general, skip fire engine control contemplates selectively skipping the firing of certain cylinders during selected firing opportunities. Thus, for example, a particular cylinder may be fired during one firing opportunity and then may be skipped during the next firing opportunity and then selectively skipped or fired during the next. This is contrasted with conventional variable displacement engine operation in which a fixed set of the cylinders are deactivated during certain low-load operating conditions.
When a cylinder is deactivated in a variable displacement engine, its valves are not actuated and although the piston typically still reciprocates, fuel is not combusted during the power stroke. Since the cylinders that are “shut down” don't deliver any net power, the proportionate load on the remaining cylinders is increased, thereby allowing the remaining cylinders to operate at an improved thermodynamic efficiency. With skip fire control, cylinders are also preferably deactivated during skipped working cycles in the sense that air is not pumped through the cylinder and no fuel is delivered and/or combusted during skipped working cycles when such valve deactivation mechanism is available. Often, no air is introduced to the deactivated cylinders during the skipped working cycles thereby reducing pumping losses. However, in other circumstances it may be desirable to trap exhaust gases within a deactivated cylinder, or to introduce, but not release air from a deactivated cylinder during selected skipped working cycles. In such circumstances, the skipped cylinder may effectively act as a gas spring. Although deactivating skipped cylinders is generally preferred, it should be appreciated that in some engines or during some working cycles it may not be possible, or in some situations desirable, to truly deactivate cylinders. When a cylinder is skipped, but not deactivated, intake gases drawn from the intake manifold are effectively pumped through the cylinder during the skipped working cycle.
Most modern passenger vehicles operate with four stroke, internal combustion engines powered by gasoline or similar fuels. These vehicles are typically equipped with one or more catalytic converters incorporated into the exhaust system to reduce environmentally harmful emissions. The catalytic converts may use a three way catalyst, which simultaneously provides sites for reducing and oxidizing chemical reactions. Specifically, the catalyst oxidizes carbon monoxide to carbon dioxide and hydrocarbons to carbon dioxide and water. It also reduces nitrogen oxides to N2 and O2. To effectively operate the catalyst must alternately be exposed to oxidizing and reducing exhaust streams, so that neither the oxidizing nor reducing sites become saturated and lose their effectiveness. This is typically done by varying the fuel/air ratio about the stoichiometric point during normal engine operation.
Although the concept of skip fire control has been around for a long time, it has not traditionally been used in the control of commercially available engines, so an additional challenge to implementing skip fire control is insuring that the engine's other engine/powertrain systems work effectively during skip fire control. One such system relates to engine diagnostics. As is well understood by those familiar with the art, modern engine management systems perform a significant amount of diagnosis of components related to engine emissions control function while the engine is operating. These diagnostic systems are often referred to as “On-Board Diagnostics” (OBD) systems and there are a number of engine diagnostic protocols that are performed while the engine is running. Modern OBD systems store and report a significant amount of information concerning the operation and state of health of various vehicle sub-systems including the powertrain. To assist in reporting, a number of standardized diagnostic monitor routines, active tests, and associated trouble codes (known as OBD-II codes) have been developed to report the detection of specific perceived malfunctions.
Many of the diagnostics protocols relate to environmental issues. Currently, many countries have regulations requiring that engines monitor and regulate emissions during use. Some jurisdictions also require periodic or continuous monitoring of the condition of various components and sensors used in the emissions control process. These may include regulations that require the testing of catalytic converters (or other emissions control devices) used in the exhaust system and/or testing of the sensors (e.g., oxygen sensors) used to monitor emissions during operation. By way of example, in the United States, there are a number of federal and state (notably California) regulations that mandate the performance of certain tests continuously, every engine on/off cycle, or at other prescribed intervals during use. One such set of regulations relating to malfunction and diagnostic systems requirements is articulated in California Code of Regulations (CCR), Title 13, Section 1968.2. Of course many other jurisdictions have their own sets of engine diagnostics requirements.
By way of example, Title 13 of the CCR, §1968.2(e)(6.2.2) requires that if a vehicle is equipped with adaptive feedback control, the OBD-II system must include a Fuel System Monitor that can detect/report a malfunction when the adaptive feedback control has used up all of the adjustment allowed by the manufacture. 1968.2(e)(6.2.1)(C) requires a Cylinder Imbalance Monitor that monitors air-fuel ratio imbalances between different cylinders. 1968.2(e)(1.2.1) requires a Catalyst Efficiency Monitor capable detecting a catalyst system malfunction when the catalyst system's conversion capability decreases to certain levels. Each of those monitors typically requires inputs from the oxygen sensors. Therefore, the efficacy of the oxygen sensors themselves must be monitored as well. In practice some of these tests can be challenging to run during normal operation of the engine, in part because specific operating conditions are desired for the execution and constraining the engine to operate in a testing mode may interfere with the driver's desired operation.
Furthermore, although existing OBD systems generally work well, several of the diagnostics algorithms are not well suited for use when an engine is operated in a skip fire manner The present application describes various techniques and protocols that are well adapted for performing and enhancing diagnostics while an engine is operated in a skip-fire operational manner